An example of a power supply apparatus for welders of this type is described in, for example, JP 2000-61636 A. The power supply apparatus disclosed in JP 2000-61636 A includes an input-side AC-to-DC converter, which converts an AC voltage to a DC voltage. The DC voltage from the converter is converted to a high-frequency voltage in an inverter. The high-frequency voltage from the inverter is voltage-transformed by a transformer. An output-side AC-to-DC converter converts the voltage-transformed high-frequency voltage to a DC voltage for application to an electrode and a workpiece. When the electrode in contact with the workpiece is separated from the workpiece, an arc is generated between the electrode and the workpiece.
Changes of an output voltage Vo applied to the electrode and the workpiece, and changes of an output current Io flowing through the electrode and the workpiece may be as shown in FIGS. 1(a) and 1(b), for example. Specifically, let it be assumed that, at a time t10, the apparatus is non-load state, where the electrode and the workpiece are separated from each other and no arc is present between them. As shown in FIG. 1(a), the output voltage Vo maintains a predetermined voltage value Va, which may be, for example, several tens of volts (V), necessary for generating an arc. On the other hand, the output current Io is zero amperes (A) as shown in FIG. 1(b).
At a time t11, which is later than the time t10, when the electrode and the workpiece are brought into contact with each other, the output current Io abruptly increases. On the other hand, the output voltage Vo decreases abruptly to about zero volts (Vo≈0 [V]). At a time t12 following the time t11, when the electrode and the workpiece are separated by some distance, an arc is generated between them. The arcing is maintained by keeping an appropriate distance between the electrode and the workpiece. In order to stabilize the arcing, the inverter is constant-current controlled so as to maintain the output current Io at a constant current value Ia. The constant-current control of the inverter also makes the output voltage Vo stable at a constant voltage value Vb. The voltage value Vb is dependent on the current value Ia of the output current Io, but, usually, it is smaller than the voltage value Va.
When the distance between the electrode and the workpiece is made larger at a time t13, which is later than the time t12, the arcing ceases, and the apparatus returns to the non-load state,
According to the described prior art, when the power supply apparatus is in the non-load state, the output voltage Vo is maintained at the constant voltage value Va necessary for starting arcing, and therefore arcing can be securedly started. However, since the inverter is driven even when the apparatus is in the non-load state, in order to maintain the output voltage Vo at the constant voltage value Va, power loss occurs. Also, since some exciting current flows through the transformer, power loss due to this exciting current occurs, too. That is, some non-load power loss represented by hatched portions 100 in FIG. 1(a) occurs. Consequently, the power consumption by the power supply apparatus as a whole increases.
An object of the present invention is to provide a power supply apparatus for welders which consumes less power than prior art welder power supply apparatus.